In general a superconducting (SC) cable must be kept at cryogenic temperatures (0-150 K or −273.25 to −123° C.) in order to function as intended/designed. Usually a cable section connects to other system components operated at ambient or elevated temperature.
In the following the general term “superconducting cable system” is used to denote a superconducting and/or hyper-conductive cable (e.g. a multi-phase, such as a three phase, cable) in combination with the relevant thermal insulation envelope.
In order to terminate a SC cable system some basic elements are normally needed, namely termination of:                1. the conductor (current element),        2. the electrical insulation (voltage element),        3. the thermal insulation (thermal element),        4. the cooling means, e.g. fluid cryogen (cooling element), and        5. optionally varying diagnostics (diagnostics element).        
Items 3), 4) and 5) are elements which are normally not present in conventional cables although 4) has some similarities to cooled bus bars and element 5) is to a certain degree also present in oil insulated cables where the oil is kept at a certain pressure that is continuously monitored. Different optional diagnostics can be implemented, e.g. monitoring of pressure, temperature (internal and/or external), flow, cryogenic fluid level, air humidity, etc.
A tri-axial HTS cable design with three concentric phases surrounded by a concentric neutral conductor (as e.g. described in US 2005173149 A (GOUGE ET AL.) 11 Aug. 2005 and in WO 2006/111170 A (NKT CABLES ULTERA) 26 Oct. 2006) have certain advantages over other HTS cable designs.
The advantages over a cold-dielectric co-axial design include:                1. Reduced use of superconducting material by 34-50% leading to reduced cost and reduced energy loss.        2. Reduced use of cryogenic envelope materials and cold surface by 30-50% leading to reduced cost and increased energy efficiency.        
Advantages compared to warm-dielectric single-phase cables include:                1. No external magnetic fields creating disturbances externally to the cable.        2. Improved relation between the electrical properties of inductance and capacitance leading to longer critical lengths, improved stability and reduced load-dependant voltage drops.        3. Reduced magnetic fields internally in the cable leading to lower energy losses and improved performance of the superconducting materials.        4. Reduced use of cryogenic envelope materials and cold surface by a factor 30-50% leading to reduced cost and increased energy efficiency.        5. Reduced number of cryogenic envelopes leading to fewer welding and fabrication steps, lower fabrication costs and increased reliability.        
Disadvantages compared to the two alternative designs may include the following:                1. Less well-known dielectric than the warm-dielectric single phase leading to higher risk in utilisation.        2. More complex cable design and termination design than the co-axial cold dielectric and the warm-dielectric single phase leading to higher risk in fabrication and in utilisation.        3. Inherently/generically imbalanced impedances in phases 1, 2, and 3.        
Advantages of HTS cables over conventional cables with conductors of copper or aluminium include normally a higher current carrying capability, reduced generation and release of heat along the cable, lower electrical loss, and lower weight.
Disadvantages compared to the conventional alternatives include normally the necessity of a cooling system, continuous thermal loss through the thermal insulation, and increased complexity of accessories such as joints and terminations.
Termination units for superconducting cable systems are discussed in a number of prior art documents.
U.S. Pat. No. 6,988,915 B (SEI) 24 Jan. 2006 deals with a terminal structure of a direct electric current superconducting cable wherein the end portions of the superconducting layers provided over a core liner are exposed in a step-by-step manner from an outer layer to an inner layer, and outgoing conductors made of a conventional conductive material are individually connected with the exposed end portions of the respective superconducting layers. An insulating fixing member supports the core and the outgoing conductors.
WO 2005/086306 A (SEI) 15 Sep. 2005 deals with a terminal structure for a multi-phase superconducting cable, wherein an electrically conductive sleeve is disposed around each of the concentrically arranged superconductive layers carrying the electrical phases and electrically connected thereto and to leads for extracting each phase at room temperature.
U.S. Pat. No. 6,936,771 B (SOUTHWIRE COMPANY) 30 Aug. 2005 deals with a termination unit for connecting a high temperature superconducting (HTS) cable immersed in pressurized liquid nitrogen to high voltage and neutral (shield) external bushings at ambient temperature and pressure. The termination unit comprises a cold housing connected to a warm housing via a transition duct wherein one or more capillary passages through or parallel to the transition duct allow gas to flow to maintain pressure equilibrium between said cold housing and said warm housing.
US 2005173149 A (GOUGE ET AL.) 11 Aug. 2005 deals with a termination unit for a superconducting cable comprising three concentrically arranged superconductive layers (tri-axial). The electrical phase conductors are terminated to copper tubes. In a preferred embodiment the tubes are concentric and separated by solid insulating tubes. The cable is cooled through liquid coolant streams inside the central tube of the cable and outside of the cable. In a preferred approach, the cold end of the termination is conduction cooled from the outside with the liquid coolant at ground potential. This requires an electrically insulating material with a high thermal conductivity.